The present invention generally relates to new and improved thermal lamination films and methods for protecting and enhancing the appearance of printed substrates. More particularly, it relates to new and improved thermal laminating films specially created for use with newer printed substrates prepared using modern digital printing methods and equipment.
Thermal laminating materials and methods are known for protecting printed substrates by adhering a protective thermoplastic polymer cover film or sheet to one or both of the major surfaces of a printed substrate. Thermal film lamination provides immediate benefits to printed materials--most notably, an attractive look and polished feel. With thermal lamination, printed materials enjoy greater durability. Unlike UV or other coatings, thermal lamination provides protection against scratching, fading and smudging. The life of the printed materials is extended in a number of ways because thermal lamination makes paper more tear resistant and protects inks on printed substrates.
Unlike other finishes, thermal lamination is a very safe and waste free process. It does not emit harmful volatiles and other substances into the atmosphere. Accordingly, thermal lamination is a significant breakthrough in finishing practices and contributes greatly toward preserving the environment. Earlier, less productive solvent-and water-based systems are now being changed over to thermal lamination. Thermal lamination is considered by many in the graphic arts community to be the finish of choice. With its high-luster look and luxurious feel, a laminated product communicates quality and creates a favorable impression with the consuming public. There are a wide variety of laminates to choose from, i.e., clear, delustered, satin finish, and glueable-stampable, to name but a few. Thermal lamination can add value to a wide range of printed materials. Illustrative applications may include: book covers, magazines, annual reports, speciality packaging, posters, photographs, gift bags, calendars, maps, menus, video boxes and trading cards, to name but a few.
Advances in the printing field, especially in digital printing resulting from the explosion in personal computers and desk top publishing, have created new problems for thermal laminating methods. More particularly, with the development of digital printing, new ink technologies are being developed. Newer inks do not dry, but instead are absorbed into the paper surface. Some glycol formulated inks do not cross-link and harden. Instead, solvent evaporates from the ink formulation so that the ink thickens but remains flexible or soft. Successfully thermal laminating these printed surfaces is difficult. Adhesion of a thermal lamination film to these printed ink surfaces is more difficult do to poor adhesion to these inks and decreased substrate surface area available for bonding.
Another recent development has been the introduction of solid inks comprising a pigment dispersed in a resin, such as polystyrene, at 100% solids. These inks may be used in colored electrostatic printer types. Heat is used to adhere the ink or fuse toner to the paper, to fuse the ink together. These solid inks are tacky when heated and, for this reason, the surface of the ink is typically sprayed with a silicone compound to prevent hot ink from sticking to rollers or other parts of printing equipment. The silicone coating makes adhesion of thermal lamination films to these printed products especially difficult.
Thermal laminating films include the base, public side protective layer, usually a clear thermoplastic polymer material, and a thermal laminating layer or adhesive layer on one side of the base layer. A thermal laminating film for newer digital printed substrates requires the use of adhesives that can be activated at lower temperatures. These adhesives must have a low viscosity, when activated, and high polarity to allow the adhesive to adhere to different ink formulations and different paper substrates, whether they are natural or synthetic.
Typical adhesives for thermal laminating films have included ethylene vinyl acetate copolymers. Usually these copolymers contain from 10 to 18% by weight of vinyl acetate units. At these lower levels of vinyl acetate content, satisfactory adhesion and performance in thermal lamination is difficult to achieve with the newer printed substrates. At higher vinyl acetate levels of about 28% or higher the thermal lamination film tends to be self welding or to block. Blocking or sticking of the film to itself when it is wound upon a roll is a very serious problem in commercial thermal lamination operations. The blocking tendency has been observed starting at about 18% by weight vinyl acetate content and the problem is only exacerbated by increasing the vinyl acetate content more than 18%. At higher levels of vinyl acetate of up to 28% or higher it is difficult to produce the films and is extremely difficult to store them. The films cause sticking to chilled casting rolls which presents a major production problem. Moreover, the edges of the rolled up film tend to fuse making unrolling and use of the material difficult, if not impossible. When large rolls are slit into narrower rolls for use, the slit rolls tend to block and stick, rendering them useless or difficult to use in commercial thermal lamination applications. Accordingly, new and improved thermal lamination films are needed which exhibit satisfactory performance and adhesion to newer digital printing substrates which can simultaneously overcome serious production and storing disadvantages previously encountered with thermal laminating films of this type.